1. Technical Field
This disclosure relates generally to the field of hydraulic switches and valves, especially spool valves, and in particular to valves for switching high and low pressure fluid supplies to a reversible pump/motor, such as for providing motive power to a passenger vehicle.
2. Description of the Related Art
According to current technology a bent-axis pump/motor machine includes a rotatable cylinder barrel having piston cylinders radially spaced around a common center. Each of the cylinders includes a piston having a first end positioned within the cylinder, and configured such that there is a pressure tight seal between the first end of the piston and the walls of the respective cylinder. A second end of each of the pistons engages a drive plate, which is coupled to a drive shaft of the machine.
The angle of the barrel can be adjusted with respect to the drive plate. It will be understood that, when the barrel and the drive plate occupy a common axis, the pistons in the barrel will not move within the cylinder as the barrel rotates. Accordingly, in this position the displacement volume of the machine is zero. On the other hand, when the axis of the barrel is rotated with respect to the axis of the drive plate, each of the pistons will reciprocate within its respective cylinder as the barrel rotates. Thus, the angle of the barrel relative to the drive plate dictates the displacement volume of the machine.
Fluid channels are coupled, via a valve plate, to the barrel, and thence to each of the cylinders of the barrel, as the barrel rotates over the valve plate.
The cylinders on one side of the barrel are coupled, via the valve plate, to a first machine port, while cylinders on the other side of the barrel are coupled to a second machine port. As the cylinder rotates over the valve plate, each cylinder is coupled first to the first port during the downstroke of the respective piston, and then to the second port during the upstroke of the piston.
The first and second machine ports are coupled to high- and low-pressure fluid sources, via a series of valves configured to selectively couple the high-pressure source to one machine port and the low pressure source to the other machine port, or alternatively, to reverse this arrangement. Such a machine may be employed as either a pump or a motor, as described below:
If the first machine port is coupled to a high-pressure fluid source, while the second machine port is coupled to a low-pressure fluid source or to a sump, the machine will exert torque in a first direction with a force that is directly related to a displacement volume of the machine, which is in turn dictated by the angle of the barrel with respect to the drive plate. Alternatively, if the high-pressure fluid source is coupled to the second machine port and the low-pressure fluid source is coupled to the first machine port, the machine will exert torque in the opposite direction, again with a force in direct proportion to the displacement volume of the machine. If the drive shaft is permitted to rotate in accordance with the applied torque, the machine will operate as a motor, providing rotational force to a transmission or some other output device. If the drive shaft is rotated against the torque applied by the machine, the machine will function as a pump, pumping fluid to the high-pressure fluid source.
Such a device is commonly referred to as a bent-axis pump/motor, and is well known in the industry.
Fluid coupling between the high- and low-pressure fluid sources and the first and second machine ports is commonly effected by first and second control valves, coupled to the first and second machine ports, respectively. Each control valve is configured to selectively couple the high- or low-pressure fluid source to its respective port. To operate in a first direction, or to apply torque in a first direction, a first one of the valves is configured to couple the high-pressure fluid source to the first machine port, while the second valve is configured to couple the low-pressure fluid source to the second machine port. To reverse the direction of applied torque of the machine, the configurations of the first and second valves is reversed, namely the first valve is configured to couple the low-pressure fluid source to the first port, while the second valve is configured to couple the high-pressure fluid source to the second port.
Bent-axis axis pump/motors of the type described above are commonly used in many applications, such as heavy construction equipment, farm machinery, and other industrial applications.
An example of a bent-axis pump/motor is described in detail in U.S. Pat. No. 4,893,549, issued to Franz Forester, which is incorporated herein by reference, in its entirety.
Some efforts have been put forth to employ hydraulic pump/motors such as the bent-axis pump/motor of the type described above, for wider use in vehicles, because of the advantages offered with respect to regenerative braking.
A hybrid powertrain vehicle that utilizes regenerative braking is described in U.S. Pat. No. 5,495,912, issued to Charles Gray, which is incorporated herein by reference, in its entirety.
Regenerative braking is a concept in which kinetic energy is reclaimed from a moving vehicle and stored for future use, rather than dissipated as heat, as is now the practice with friction brakes commonly used in motor vehicles. According to the concept of regenerative braking, when an operator applies a brake to slow a vehicle, the wheels of the vehicle are coupled to an energy collection device such as an electric generator or a hydraulic pump. As the brake is applied, the generator or pump draws energy from the rotating wheels and stores that energy in a storage medium. In turn, the resistance provided by the energy collection device slows the vehicle. In many respects, a hydraulic pump/motor is an ideal device for such an application, inasmuch as the device may be converted from a drive motor to a pump for braking simply by reversing the polarity of the first and second machine ports. Additionally, the braking force can be regulated by adjusting the angle of the cylinder barrel, as described above.
However, several problems have been encountered in developing such an application. First, valves of the type used to switch the pumps must have a large fluid capacity to accommodate the volume of fluid used by the pump/motor at full displacement. Such valves may require significant actuation energy, and are often subject to fluid leakage. Any fluid escaping from a high-pressure fluid circuit represents a loss of energy or fuel economy.
Second, when hydraulic pump/motors are used in passenger vehicles that are intended to travel at freeway speeds, the speed with which the machine can switch from motor to pump becomes critical, inasmuch as this represents the lag time between the moment an operator applies the brake and the moment the vehicle begins braking. This means that the switching valves must be very responsive and able to change modes quickly.
Third, there are safety issues that must be addressed with respect to a pump/motor. For example, if a vehicle operator were to apply the brake in a vehicle employing a conventional pump/motor, the polarity of the pressure circuits of the motor would be reversed, causing the motor to exert torque in opposition to the direction of rotation of the wheels of the vehicle, slowing and eventually stopping the vehicle. However, if the operator continues to apply the brake once the vehicle has stopped, the pump/motor will begin rotating in reverse, causing the vehicle to travel in reverse. A normal response on the part of an operator in such a situation would be to press harder on the brake in an effort to stop the vehicle. This would cause the angle of the pump/motor to increase, thereby accelerating the travel in reverse.
Addressing each of these issues adds complexity to the motor. As the system becomes more complex, manufacturing costs increase and the potential for failure of a component increases. With this increased potential come additional safety concerns, which must be addressed before such a device can be employed in passenger vehicles.